I bought a 98 speedster with twin 787s with a blown engine. I replaced one engine and rebuilt the carbs on that engine. After getting everything back together, it was fine starting it out of the water. Once in the water, that engine wouldn't rev past about 2500rpm and started to fall off. Everything is fine electronically. I first thought maybe I had a weak fuel pump so I switched sides with the fuel pumps, no difference. I switched the carbs to the other engine that was running fine and now it has the exact same problems, won't rev past 2500 rpms. I can rev past 2500rpms by feathering the choke and keeping the choke slightly on can run at 7000rpm. It seems like the carbs keep running out of fuel. I've taken the carbs back apart and checked everything again. I've pressure tested the pop-offs and they are in spec, on the light side actually. I've also tried adjusting both high-speed and low-speed adjustment screws. Any help would be greatly appreciated. Thanks
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98 speedster, one engine won't rev past 2500 without slightly pulling choke
- Thread starter blazn1320
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Dr Honda
Well-Known Member
A couple things first.....
1) Welcome :thumbsup:
2) STOP RUNNING THE BOAT !!!!!!!!!!!!
What you are describing is exactly what you say it is. It's running out of fuel. AND... since it's a 2-stroke engine... you WILL kill the engine !!! The problem is 2 fold... first... the pistons are cooled via the fuel, and when it goes lean... you can melt a piston. Second, the oil is transported via the fuel. So, when you go lean... you are running out of oil too.
OK... with that out of the way... let's work on the problem.
a) When you "rebuilt" the carbs, what did you do?
b) Did you replace any parts?
c) Were the replacements OEM/Mikuni or aftermarket like Windrosa?
d) Did you replace any of the pump parts? If so, did you poke holes in the rubber pins that hold the valves?
e) Did you verify that the high speed check valve is flat, and sealing?
f) did you verify 100% that the bypass ports are open?
g) Did you verify 100% that they held pressure?
h) Did you change the spring on the Needle and seat?
I) Did you make sure the needle arm was (more or less) flush?
j) do you have the OEM airbox on, with the screen?
k) Did you check the pop-off with a little oil down in the needle?
l) Did you replace the O-rings on the high and low needles?
m) did you make sure to set the high and low needles to spec? (and not just where they were when you took them apart)
o) did you make sure all the carb jets were right? (it's new to you, maybe someone messed with them)
OK... here's the deal...
There are a few things that can cause the issue you are having.
1) If the high speed check valve is leaking, you will suck the fuel out of the diaphragm, and when you open the throttle, there simply isn't any fuel left. if you "Pop" the choke, it will force the system to feed fuel (suck it in actually) and as long as your RPM's are high... it will normally keep running until you throttle back.
2) If you don't have the OEM air box on... you don't have the proper "Signal" to the carbs, and they wont' run properly. It's not very restrictive... but these carbs spit a lot, and that mesh screen helps hold the fuel in. Aftermarket filters SUCK, and are hard to tune. (I don't recommend any of them on a recreational toy)
3) The aftermarket parts all suck. (other than the gaskets) The aftermarket diaphragms don't work, and they will cause all kinds of issues. Normally hard starting... but they can keep the needle from opening too. OEM Mikuni have a red anodized nipple, so they are easy to spot.
4) Pop-off is just a test, and it's not perfect. The reason is... in the carb... the needle doesn't open with pressure... it opens with the vac from the engine. SO... if you replaced the spring, it may be too heavy. AND, if you replaced the Needle and seat with a larger size... that will be exactly why it's not running.
Anyway.. that's a long list... but I'm sure the answer is there. Oh... and running the boat on the trailer wont' tell you anything. Without loading the engine... you cant figure it out. (so don't bother with running dry) AND... it's very hard on the carbon seal on your driveshaft. It's cooled/lubed via the lake water. When running on the trailer... it's just wearing out.
1) Welcome :thumbsup:
2) STOP RUNNING THE BOAT !!!!!!!!!!!!
What you are describing is exactly what you say it is. It's running out of fuel. AND... since it's a 2-stroke engine... you WILL kill the engine !!! The problem is 2 fold... first... the pistons are cooled via the fuel, and when it goes lean... you can melt a piston. Second, the oil is transported via the fuel. So, when you go lean... you are running out of oil too.
OK... with that out of the way... let's work on the problem.
a) When you "rebuilt" the carbs, what did you do?
b) Did you replace any parts?
c) Were the replacements OEM/Mikuni or aftermarket like Windrosa?
d) Did you replace any of the pump parts? If so, did you poke holes in the rubber pins that hold the valves?
e) Did you verify that the high speed check valve is flat, and sealing?
f) did you verify 100% that the bypass ports are open?
g) Did you verify 100% that they held pressure?
h) Did you change the spring on the Needle and seat?
I) Did you make sure the needle arm was (more or less) flush?
j) do you have the OEM airbox on, with the screen?
k) Did you check the pop-off with a little oil down in the needle?
l) Did you replace the O-rings on the high and low needles?
m) did you make sure to set the high and low needles to spec? (and not just where they were when you took them apart)
o) did you make sure all the carb jets were right? (it's new to you, maybe someone messed with them)
OK... here's the deal...
There are a few things that can cause the issue you are having.
1) If the high speed check valve is leaking, you will suck the fuel out of the diaphragm, and when you open the throttle, there simply isn't any fuel left. if you "Pop" the choke, it will force the system to feed fuel (suck it in actually) and as long as your RPM's are high... it will normally keep running until you throttle back.
2) If you don't have the OEM air box on... you don't have the proper "Signal" to the carbs, and they wont' run properly. It's not very restrictive... but these carbs spit a lot, and that mesh screen helps hold the fuel in. Aftermarket filters SUCK, and are hard to tune. (I don't recommend any of them on a recreational toy)
3) The aftermarket parts all suck. (other than the gaskets) The aftermarket diaphragms don't work, and they will cause all kinds of issues. Normally hard starting... but they can keep the needle from opening too. OEM Mikuni have a red anodized nipple, so they are easy to spot.
4) Pop-off is just a test, and it's not perfect. The reason is... in the carb... the needle doesn't open with pressure... it opens with the vac from the engine. SO... if you replaced the spring, it may be too heavy. AND, if you replaced the Needle and seat with a larger size... that will be exactly why it's not running.
Anyway.. that's a long list... but I'm sure the answer is there. Oh... and running the boat on the trailer wont' tell you anything. Without loading the engine... you cant figure it out. (so don't bother with running dry) AND... it's very hard on the carbon seal on your driveshaft. It's cooled/lubed via the lake water. When running on the trailer... it's just wearing out.
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I'd like to add that carb work to me is more clockwork than other mechanical work. You must work in a clean surface, and there are many verifications required, and everything must be right, to prevent having to reassemble all, and then take it all apart again after realizing it did not work.
I'd love to say, hang in there and you will figure it out. I did not get there. I have twin 717, and when I need a carb rebuild, I now just buy new carbs for $200 each or less (mine are single carb intakes) a professional rebuild with all paid is around $175. I have not found cheaper, so I end up buying new. I never got rebuilds to be perfect, and I just tried again a month or two ago with some old ones I had stored. about 80% in, I figured with how clean and nice everything needs to be, and with the likelihood the job was not perfect, was not worth a wasted weekend if the rebuild has the slightest problem (which has happened to me before, I've had the boat for 13+ years after all), so I stopped the rebuild of carbs, and got a spare set.
I don't want to discourage you, but instead share an out if you reach the point where you want to give up because the expense of your time and rebuild parts, and rework, exceed the value you may get from buying new or paying someone to do it right, if you give it your best shot and can't get it right. I do feel a bit stomped, but my boating weekend and the time to remove and replace have resulted in situation where DIY on this job has not been a value proposition for me. And for me, that's OK.
Do you best, but sometimes you win by admitting loss, and changing direction.
Best of Luck!!!
I'd love to say, hang in there and you will figure it out. I did not get there. I have twin 717, and when I need a carb rebuild, I now just buy new carbs for $200 each or less (mine are single carb intakes) a professional rebuild with all paid is around $175. I have not found cheaper, so I end up buying new. I never got rebuilds to be perfect, and I just tried again a month or two ago with some old ones I had stored. about 80% in, I figured with how clean and nice everything needs to be, and with the likelihood the job was not perfect, was not worth a wasted weekend if the rebuild has the slightest problem (which has happened to me before, I've had the boat for 13+ years after all), so I stopped the rebuild of carbs, and got a spare set.
I don't want to discourage you, but instead share an out if you reach the point where you want to give up because the expense of your time and rebuild parts, and rework, exceed the value you may get from buying new or paying someone to do it right, if you give it your best shot and can't get it right. I do feel a bit stomped, but my boating weekend and the time to remove and replace have resulted in situation where DIY on this job has not been a value proposition for me. And for me, that's OK.
Do you best, but sometimes you win by admitting loss, and changing direction.
Best of Luck!!!
Dr. Honda, thanks for all your help.
To answer your questions...
a-c) When I rebuilt the carbs, I did not replace any parts. I only replaced gaskets, diaphragm, o rings , and pop off springs. I don't know the brand of the kits, bought them from a local supplier and they had generic packaging.
d) If by pump parts, you mean fuel pumps, I don't have these. My pumps are separate from the carbs.
e) By high speed check valve, I am thinking you are talking about the little clear plastic flap above the pilot jet. I replaced these and they appear flush.
f-g) To be quite honest as I am new to this, I am not sure what the bypass ports are. Are you talking about the return fuel ports?
h) I did change the springs and I have a pressure tester. I've even tried a lighter spring at the suggestion of one of my local jet ski mechanics. That made no difference.
i) Needle arms are flush.
j) Problem is prevalent with or without airbox and with or without screen. No noticeable difference either way.
k) I did check needle valves wet and they hold pressure without leaking.
l) O rings were replaced on both needles.
m) Needles were adjusted to factory spec. 0 turns on high and one and a half on low. I have tried making adjustments to both and this makes no difference to problem.
n) I have just checked carb jets low are 67.5 and high are 142.5 mag and 145 pto which is all factory size.
1. I will recheck the check valves.
2. The airboxes are oem.
3.I'm starting to think this may be the made in china rebuild kits. The diaphragms do not have the red pins like the Mikuni, they instead have a round metal tab. This could possibly be keeping the needle valves from opening far enough and not letting enough fuel in. I went ahead and purchased the real Mikuni rebuild kits this morning. They are obviously not nearly as cheap as the other ones I bought, but the extra money would have been well spent before if that's what this is. Here is a pic of the diaphragm.
To answer your questions...
a-c) When I rebuilt the carbs, I did not replace any parts. I only replaced gaskets, diaphragm, o rings , and pop off springs. I don't know the brand of the kits, bought them from a local supplier and they had generic packaging.
d) If by pump parts, you mean fuel pumps, I don't have these. My pumps are separate from the carbs.
e) By high speed check valve, I am thinking you are talking about the little clear plastic flap above the pilot jet. I replaced these and they appear flush.
f-g) To be quite honest as I am new to this, I am not sure what the bypass ports are. Are you talking about the return fuel ports?
h) I did change the springs and I have a pressure tester. I've even tried a lighter spring at the suggestion of one of my local jet ski mechanics. That made no difference.
i) Needle arms are flush.
j) Problem is prevalent with or without airbox and with or without screen. No noticeable difference either way.
k) I did check needle valves wet and they hold pressure without leaking.
l) O rings were replaced on both needles.
m) Needles were adjusted to factory spec. 0 turns on high and one and a half on low. I have tried making adjustments to both and this makes no difference to problem.
n) I have just checked carb jets low are 67.5 and high are 142.5 mag and 145 pto which is all factory size.
1. I will recheck the check valves.
2. The airboxes are oem.
3.I'm starting to think this may be the made in china rebuild kits. The diaphragms do not have the red pins like the Mikuni, they instead have a round metal tab. This could possibly be keeping the needle valves from opening far enough and not letting enough fuel in. I went ahead and purchased the real Mikuni rebuild kits this morning. They are obviously not nearly as cheap as the other ones I bought, but the extra money would have been well spent before if that's what this is. Here is a pic of the diaphragm.
Dr Honda
Well-Known Member
Responses in red....
Those diaphragms SUCK. The biggest issue is hard restarting, because the actually "Bellows" on the edge is too tall. AND... will hold the needle open slightly. BUT... they also are known to hold it shut. Get a couple Mikuni diaphragms, needles/seats, and springs... and your issue may go away.
Those diaphragms SUCK. The biggest issue is hard restarting, because the actually "Bellows" on the edge is too tall. AND... will hold the needle open slightly. BUT... they also are known to hold it shut. Get a couple Mikuni diaphragms, needles/seats, and springs... and your issue may go away.
To the OP
Just so you know you cannot get a rebuild kit for the divorced fuel pumps and the internal parts are not the same as the one that are built into the carbs. When you open the pump the gasket will stretch like taffy and not return to the same shape. Many guys use a mikuni pump that is hexagon shaped and it has a 54 in the part number iirc. Only problem is that it is a dual pump so you need to block the one outlet. JHjesse did it on here and has pics somewhere. If I find the link I'll post it up.
Just so you know you cannot get a rebuild kit for the divorced fuel pumps and the internal parts are not the same as the one that are built into the carbs. When you open the pump the gasket will stretch like taffy and not return to the same shape. Many guys use a mikuni pump that is hexagon shaped and it has a 54 in the part number iirc. Only problem is that it is a dual pump so you need to block the one outlet. JHjesse did it on here and has pics somewhere. If I find the link I'll post it up.
I think u can still find places that have the oem 787 external pumps for the boats. Aftermarkets are cheaper and rebuildable. The 717 no luck I did the dual aftermarket mikunis and jbwelded and capped one output. Simple as pie just wait for the jbweld to dry.
You can usually tell when you take your carbs apart if they are going to be a pain to clean. You will have white power all over them and some pitting sometimes in the carbs. I been lucky all the one I have done even all chalked up still clean and run good.
U can use a sonic cleaner from like harbor freight to help clean the carbs to. Use some purple stuff and water to to clean up. Can use harsher stuff also just need to remove all rubbers and plastic parts.
U can also get good deals on new carbs also like others have mentioned. Unless you do work it could cost you the same. When I work on blower and weed eaters I just buy a new carb when it needs a rebuild they are so small and hard to clean and the price difference is like 5 dollars.
U will get it working. Follow what everyone said.
You can usually tell when you take your carbs apart if they are going to be a pain to clean. You will have white power all over them and some pitting sometimes in the carbs. I been lucky all the one I have done even all chalked up still clean and run good.
U can use a sonic cleaner from like harbor freight to help clean the carbs to. Use some purple stuff and water to to clean up. Can use harsher stuff also just need to remove all rubbers and plastic parts.
U can also get good deals on new carbs also like others have mentioned. Unless you do work it could cost you the same. When I work on blower and weed eaters I just buy a new carb when it needs a rebuild they are so small and hard to clean and the price difference is like 5 dollars.
U will get it working. Follow what everyone said.
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